Wildfire Simulation Technology – Part 3

Beneath every wildfire model there is a set of assumptions, ideally based on a theory of how fire ignites and spreads. The model, however, is only as good as the theory that drives it and the data that goes into it. As computers have gotten smaller and faster, models have become more powerful. However, the fire simulations are heavily dependent on the assumptions made by a particular model. This last installment on wildfire simulation software takes a hard look at the current thinking on this aspect of fire science in the U.S.

Mark Finney, a research forester at U.S. Forest Service (USFS) in Missoula, MT, and one of the foremost authorities on how assumptions on wildfire apply to models, provides some background material on how we arrived at our current thinking in fire science. “There were a lot of basic studies done 50 years ago trying to understand wildland fire from an engineering perspective, but since wildland fire is an applied discipline, there’s a lot of interest in coming up with practical products that you can use,” he explained. “Whereas people would normally try to understand what’s going on and then model it once they had a theory, there actually is no theory to fire spread.”

The Science of Combustion and Spread

Despite appearances, fire combustion and spread are not simple phenomena. “There are a lot of variables and a number of things that cannot be known very well when you try to model something. But when you look at very simple experiments in laboratory conditions, we still can’t explain exactly the physics going on there,” said Finney. “It’s been recognized for a long time that wildland fire spread is a process of repeated ignitions. If the sequence of ignitions fails, then you don’t have spread.”

Finney contends that people have jumped right into determining the rate of spread rather than trying to better understand the process of ignition due to the need for operational kinds of tools (i.e. models). “They only modeled the conditions under which spread occurred, which is only part of the equation because most of the interesting stuff is ‘When is it going to spread, when is it not going to spread and what are the various factors that contribute to that threshold?’” he explained.

“You have all kinds of things (from moisture content to spacing between the fuel elements), so there are so many parameters that can contribute in different ways in different combinations (like weather, wind and turbulence) to produce an ignition that it is complicated, but you can still understand what the requirements are for ignition,” Finney continued. “Our approach to this is not to model fire, but to understand the physics of fire spread. There are some very interesting things that come out of this that are exactly the opposite of what most models assume, which raises some questions.”

And there are some things that simply may not be possible. “It is hard to extract the process you’re interested in without making it so out of context that it isn’t relevant to what you were originally studying, for instance keeping out how radiation or convection work in a wildland fire is tough because they’re always there,” said Finney.

Running experiments on combustion and spread is not only time consuming, it’s a bit pricey. “At the moment, running an experiment with fire in a laboratory is expensive, uncertain, and you can find almost no one to back it,” said Finney. “We have pretty good facilities here [at USFS], but it’s very expensive to have the kinds of technical people here – all kinds of engineers, machinists, and technicians – to run an experimental program.”

Look Before You Leap

Proceeding with a simulation without having a good understanding of all the parameters of a fire can be risky, especially with lives and property at stake. “What we’ve found is that people jump straight into modeling without having a full understanding of what’s going on,” Finney continued. “It seems like models were pursued, not as a means of understanding the phenomena so much, but in order to produce something that could be implemented in an operational way and that kind of shortcut the understanding part.”

Modeling is easy, according to Finney, anybody can produce a model – so there’s dozens of models. “But, that’s a telltale sign of when people are substituting technology for science,” he warned. “We lack that underlying knowledge of how things really work. We’re not yet at the point where we know what we can put in a model, so people are stuffing whatever they want in there.” Not something that makes for reliable wildfire behavior prognostications!

An Abundance of Theories

Not everyone in fire science is working from the same set of notes. “There’s not one way to look at this that people agree on, and that’s a problem because there’s only one physics somewhere and until you can explain how fire spreads and can repeatedly prove it, it’s really hard to formulate the principles on which you’re going to base your model,” Finney explained. “We are still at the point where scientists don’t have a coherent theory of how fire spreads, and even worse, they don’t even bother with the theory.” Finney says scientists assume radiation works like this and assume convection works like that, and because they’re trying to model it rather than try to understand it, there’s a real difference.

Finney is concerned that the cart is being put before the horse. “Models are an abstraction of reality; they’re a simplification and are intended to be wrong because you have to compromise things to make a model work. So, it is a little surprising at this stage of the game that we don’t have a theory of fire spread.”

According to Finney, we haven’t yet reached the point where we have a reliable physical theory and where our models all actually diverge from that theory. “We’ve narrowed in on some things that, although they don’t constitute a complete theory, they are part of one, very essential physical process,” he explained.

Practical Application

As mentioned in a previous article in this series, a wildfire that burned outside of Hungry Horse, MT, in 2003 demonstrated the power (and the limitations) of modeling. Finney explained: “We prepared a number of different model runs [for firefighters], one of which was a thunderstorm that produced spotting on the other side of the lake, but it was enough to say, ‘Hey, this could happen,’ so that they were able to take precautions against it. The thunderstorm appeared, it blasted some embers across the lake, and they were there to pick them up.” Hungry Horse was an example of where a model could be used to point out something that wasn’t obvious, concluded Finney – not predicting that it would happen, just showing that it could happen.

“Models show the potential for something to happen, but it takes awhile for operational people to understand how to use models,” said Finney. “It takes awhile for people to understand that this software won’t predict the results of tomorrow’s horse race.”

And bear in mind the limitations. “One thing that really constrains us with fire is that we’re never going to know enough about the environment (what the wind is like everywhere or what the fuel particles are doing everywhere), so we can’t feed even a perfect model with enough information,” Finney cautioned.

So, it appears that much remains to be done before wildfire models will be able to accurately forecast how a blaze behaves, but each day fire scientists get closer to realizing that goal. It will be interesting to see where the U.S. Forest Service scientists are in another 50 years!

MIKE ARCHER is an author, wildfire consultant, systems engineer, and public speaker who has been interviewed by CBS News, KABC-TV, USA Today, and the Associated Press on wildfire topics, and has been part of a delegation testifying before government bodies (including Congress and the California Senate) on fire-related issues. He runs the Wildfire News of the Day blog and Firebomber Publications.